1. Field of the Invention
The present invention relates to an optical device with one or more photodetector regions therein, and an optical module having a single optical fiber or a plurality of optical fibers (optical fiber array), or a single optical waveguide or a plurality of optical waveguides. More particularly, the present invention concerns an optical device and an optical module, which are suitable for monitoring signal light while the signal light is propagated through an optical transmitting means.
2. Description of the Related Art
In present optical communication technology, it is important to monitor communication quality. Particularly, monitoring of optical output plays an important role in the field of wavelength multiplex communication technology.
In recent years, with respect to optical output monitoring technology, there have been growing demands for smaller size, higher performance, and lower cost. Attention has been attracted to a TAP (branching coupler for monitoring an input signal) system, having a slit structure provided directly within the optical fibers or optical waveguides, for extracting and detecting a portion of the signal light in order to monitor signal light quality.
Heretofore, a technique disclosed in Japanese Laid-Open Patent Publication No. 2001-264594, for example, has been proposed. According to the disclosed technique, an optical fiber is placed in a V-shaped groove defined in a glass substrate, and thereafter a parallel groove is defined in the glass substrate obliquely across the optical fiber (the optical axis thereof). A light branching member is inserted into the parallel groove, and an ultraviolet-curable resin (adhesive) is filled in the gap between the light branching member and the groove wall.
From among the signal light that is propagated through the optical fiber, a light component (branched light) branched by the light branching member is extracted out of the cladding. The branched light is detected by a photodetector device, for example, in order to monitor the signal light.
The optical module with the above TAP system must employ a photodetector device, and the branched light should be applied to the photodetector device without degradation in characteristics.
In an in-line optical module having a plurality of optical fibers or optical waveguides in a horizontal array, it is essential to employ photodetector devices having a plurality of photodetector regions corresponding to the optical fibers or optical waveguides. Since the photodetector devices are disposed closely together, they are susceptible to stray light, and when a photocurrent (carrier) generated in each of the photodetector regions partially flows as a leakage current to a common electrode through adjacent or other photodetector regions, crosstalk characteristics tend to be lowered.
As shown in FIG. 13, a conventional photodetector device 100 for detecting light applied to its reverse side has a photodetector region 104 on the surface of a light-transmissive base 102. The reverse side of the base 102 (to which light is applied) is coated with a single film (single-layer film) 106 of highly moisture-resistant SiN or the like for passivation and anti-reflection.
Light is normally applied to the photodetector device 100 perpendicularly to the reverse side of the base 102. When light is applied perpendicularly to the reverse side of the base 102, light can be applied to the photodetector region 104 essentially without changing characteristics of the light at a certain wavelength, e.g., reflection characteristics and polarization dependent loss (PDC: Polarization Dependent Current), simply by changing the thickness of the single-layer film 106.
However, the optical module of the TAP system, which has a slit defined in the optical fibers or the optical waveguides, emits branched light 108 obliquely from the optical fibers or the optical waveguides. In order to apply the branched light 108 perpendicularly to the photodetector region 104 of the photodetector device 100, the photodetector device 100 has to be mounted in place, such that the surface to which the branched light 108 is applied is inclined. When it is mounted in place, it is difficult to control the angle of the photodetector device 100. Therefore, the mounting cost is high, and the optical module tends to become large in size.
The photodetector device 100 may possibly be mounted in place such that the surface to which the branched light 108 is applied lies horizontally, i.e., parallel to the optical axis of the light that passes through the optical fibers or the optical waveguides. With such a configuration, however, the branched light 108 is applied obliquely to the reverse side of the photodetector device 100, with the result that the characteristics of light at a certain wavelength, e.g., reflection characteristics and polarization dependent loss (PDC), vary greatly. Detection accuracy of the branched light 108, i.e., the accuracy at which the signal light can be monitored, is thus reduced.